The films known as metallised ‘holographic’ films are well-known and are used, in particular, for the marking of products or of official documents such as identity cards, passports, credit cards, cheques, banknotes, travel tickets, entrance tickets, cards giving an entitlement to various services, etc. Such a ‘holographic’ marking makes it possible, in particular, to make the product or document secure by identifying it and/or guaranteeing its authenticity and/or preventing its alteration and/or preventing its reproduction.
Such known holographic films may take the form of films on a reel to be cut off at the time of their application onto the product or document to be protected, pre-cut films in the form of vignettes to be integrated into the manufacture of a product or document, self-adhesive labels, or transfer films enabling the transfer of a ‘holographic’ mark onto a substrate to be marked (FR 2 897 556).
The known metallised holographic films include one or more transparent layers, at least one of which is embossed in such a way as to exhibit at least one motif that is capable of forming at least one image, named holographic image, which is visible by showing through and by diffraction. Throughout the text, by the term ‘holographic’ any image is designated that is formed by diffraction by a structure in relief.
In traditional manner these transparent layers include a polyester backing, possibly a detachment layer if it is a question of a transfer, and a embossing layer, the embossed motif being created on the face of this embossing layer opposite the backing. Various known processes may be used in order to create this embossed motif: application of a stamping die, embossing, engraving, beam of particles etc.
In order to impart a reflective metallised appearance to at least one portion of the motif, at least one reflective metallised layer is applied facing and in the format of said portion. Such a reflective metallised layer may be an HRI (High Reflex Index) layer, created by vacuum deposition by vaporisation of zinc sulfide and/or aluminium sulfide.
Then a step of selective subtractive dissolution of at least one such reflective metallised layer is carried out, in order to form motifs constituted by delimited portions of metallised holographic image(s), in particular when said reflective metallised layer is opaque. In order to do this, a layer of protective varnish is applied facing the portions that have to stay metallised, and the film is subjected to a chemical attack, for example by immersion in a corrosive bath such as a bath of sodium hydroxide at elevated temperature. After rinsing and neutralisation in an acid bath, the holographic film is obtained which is metallised facing certain portions (in general, opaque if the metallised layer is opaque), and transparent facing the other, non-metallised portions. This known process is the only one that enables metallised holographic motifs of relatively fine resolution to be obtained, in particular graphics or texts of maximum line width of the order of 0.6 mm, or motifs ornamented with guilloche of maximum line width of the order of 0.1 mm.
In a known variant, firstly a continuous HRI layer is applied which remains at least partially transparent facing all the holographic images and substantially in the format of the embossed layer, then a layer of opaque metallic deposit delimited by selective subtractive dissolution is applied, in order to form motifs as indicated above.
These manufacturing processes are costly, require complex and costly specific plants, involve carrying out several (at least four) successive steps, and are long and difficult to implement (vacuum deposition and chemical attack).
These processes cannot be implemented at sites in industrially underdeveloped countries. Now it would be advantageous to be able to carry out the manufacture of holographic markings, in particular for purposes of security, as close as possible to the sites of use, and not only for reasons of economy and respect for the environment but also in order to avoid the intervention of multiple persons, delays and transportation, likely to constitute risks as regards security.
In addition, these steps of vacuum deposition and chemical attack are not respectful of the environment and require a specific and costly management of waste. In particular, it should be noted that the steps making it possible to carry out the chemical attack bring about washing water and rinsing water containing toxic metallic substances which themselves have to be the subject of purification treatments.
Furthermore, the steps of chemical attack are likely to dissolve or damage not only the unprotected portions of the transparent metallised layer but also the possible underlying layers, for example portions of an HRI reflective layer formed from a deposit of zinc sulfide, and/or other inscriptions on the security device possibly incorporated beforehand. Consequently, the known processes for manufacture of holographic films are not very compatible with the preparation of complex films exhibiting a plurality of different devices providing a plurality of distinct security functions.
The films obtained are therefore also costly and limited in their applications. In particular, it should be noted that a significant part of the raw material being used for the manufacture of the films is rejected in the form of waste (portions of the metallised layer dissolved by chemical attack).
In addition, the reflective metallised motifs capable of being created by the delimitation of the portions of metallised holographic image(s) cannot exhibit very great fineness in width and/or line width, nor complex and contorted shapes incompatible with the chemical attack carried out subsequently.
Furthermore, in another variant described in U.S. Pat. No. 5,629,068 a layer of metallic-effect ink, capable of containing metallic lamellae, is applied directly onto an embossed layer. In the process for manufacture described in U.S. Pat. No. 5,629,068, the metallic lamellae have to be very fine and of sufficiently significant length and width in order to be able to connect the peaks of the motifs in relief of the embossed layer. The reflective metallised motifs thus created are, however, limited in terms of holographic effects and applications.
The invention aims to overcome these disadvantages.